Ultrafine particulate titanium oxide has been widely employed as, for example, an ultraviolet shielding material, an additive for silicone rubber, a dielectric raw material, and a component of a cosmetic composition (as used herein, the expression “titanium oxide,” which is commonly used to represent oxides of titanium, encompasses “titanium dioxide” as specified in Japanese Industrial Standards (JIS)). Titanium oxide is also employed in, for example, photocatalysts and solar cells.
Titanium oxide assumes any of the following three crystal forms: rutile, anatase, and brookite. Anatase- or brookite-type titanium oxide, which exhibits excellent photoelectrochemical activity, is employed in photocatalysts and solar cells, rather than rutile-type titanium oxide.
By virtue of its photocatalytic activity, titanium oxide is employed in, for example, antibacterial tile, self-cleaning structural materials, and deodorant fibers for decomposition of organic substances. The mechanism by which titanium oxide decomposes organic substances is as follows. Titanium oxide absorbs ultraviolet rays, to thereby generate electrons and holes therein. The thus-generated holes react with water adsorbed onto titanium oxide, to thereby form hydroxy radicals. The thus-formed radicals decompose, into carbon dioxide gas and water, organic substances that have adsorbed onto the surfaces of titanium oxide particles (“Hikari Kuriin Kakumei” authored by Akira Fujishima, Kazuhito Hashimoto, and Toshiya Watanabe, CMC Publishing Co., Ltd., pp 143-145, (1997)). In titanium oxide exhibiting high photocatalytic activity, holes are readily generated, and the thus-generated holes readily reach the surface of the titanium oxide. According to “Sanka Chitan Hikari Shokubai no Subete” (edited by Kazuhito Hashimoto and Akira Fujishima, CMC Publishing Co., Ltd., pp 29-30, (1998)), anatase-type titanium oxide, titanium oxide having small amounts of lattice defects, or titanium oxide having small particle size and large specific surface area exhibits high photocatalytic activity.
Studies on application of titanium oxide to solar cells have been performed since 1991, when Graezel, et al. of Ecole Polytechnique Federale de Lausanne reported a dye-sensitized solar cell which employs titanium oxide in combination with a ruthenium-based dye (M. Graezel, Nature, 353, 737, (1991)). In the aforementioned dye-sensitized solar cell, titanium oxide serves as a dye carrier and as an n-type semiconductor, and is employed in a dye electrode bound to an electrically conductive glass electrode. The dye-sensitized solar cell has a structure such that an electrolyte layer is sandwiched between the dye electrode and a counter electrode. In the solar cell, electrons and holes are generated through absorption of light by the dye. The thus-generated electrons reach the electrically conductive glass electrode via a titanium oxide layer, and are discharged to the outside of the glass electrode. Meanwhile, the above-generated holes are conveyed to the counter electrode via the electrolyte layer, and are bound to the electrons supplied through the electrically conductive glass electrode. In order to improve properties of such a dye-sensitized solar cell, titanium oxide must be readily bound to a dye. Japanese Patent Application Laid-Open (kokai) No. 10-255863 describes that anatase-type titanium oxide is readily bound to a dye, and Japanese Patent Application Laid-Open (kokai) No. 2000-340269 describes that brookite-type titanium oxide is suitable for use in a dye-sensitized solar cell.
Functions of titanium oxide are more fully benefited from titanium oxide of high dispersibility. Titanium oxide of low dispersibility exhibits high hiding power. Therefore, when titanium oxide of low dispersibility is employed in a photocatalyst, a limitation is imposed on use of the photocatalyst. When titanium oxide of low dispersibility is employed in the field of solar cells, since such titanium oxide tends not to transmit light, the amount of light absorbed in the titanium oxide is lowered, whereby photoelectric conversion efficiency is lowered. In general, titanium oxide having a particle size of about ½ the wavelength of visible light exhibits maximum light scattering amount (hiding power), and the light scattering amount is lowered in accordance with a decrease in particle size (“Titanium Oxide” authored by Manabu Kiyono, Gihodo Co., Ltd., p. 129, (1991)). In many cases, titanium oxide having a primary particle size of some nm to some tens of nm is employed in the aforementioned technical field, and therefore, titanium oxide with excellent dispersibility scatters low amounts of light. However, titanium oxide exhibiting low dispersibility and having large size of aggregated particles exhibits increased light scattering.
Therefore, titanium oxide employed in the aforementioned technical field must exhibit high dispersibility, and thus ultrafine particulate titanium oxide of anatase- or brookite type, which exhibits high dispersibility, is employed in the art. Although not clearly defined, the primary particle size of ultrafine particles is generally about 0.1 μm or less.
In the case where titanium oxide is employed in a photocatalyst or a solar cell, when a corrosive component such as chlorine is present in the titanium oxide, a substrate is corroded or denatured. Therefore, the chlorine content of titanium oxide must be lowered. Desirably, the amount of Fe, Al, Si, or S in titanium oxide is reduced. For example, when the Fe content of titanium oxide is excessively high, the titanium oxide is colored, and the thus-colored titanium oxide is not suitable for use in a material requiring transparency. Meanwhile, when the amount of component of titanium oxide particles, such as Al or S, is excessively large, lattice defects are generated in the particles. When such titanium oxide particles are employed in a photocatalyst or a solar cell, functions thereof may be impaired.
Production processes for titanium oxide are roughly classified into two types: a liquid-phase process in which titanium tetrachloride or titanyl sulfate is hydrolyzed; and a vapor-phase process in which titanium tetrachloride is reacted with an oxidative gas such as oxygen or steam. Titanium oxide produced through the liquid-phase process contains anatase as a primary phase, but assumes the form of sol or slurry. A limitation is imposed on the use of titanium oxide in the form of sol or slurry. When such titanium oxide is to be employed in the form of powder, the titanium oxide must be dried and aggregation increases along with progress of drying a titanium oxide powder which has been wetted with a solvent [“Ultrafine Particle Handbook” edited by Shinroku Saito, Fujitec Corporation, p 388, (1990)]. When the titanium oxide powder is employed in, for example, a photocatalyst, the titanium oxide powder must be considerably crushed or pulverized in order to enhance its dispersibility. Such a pulverization treatment may cause contamination of the titanium oxide powder with wear products, along with variation in the particle size of the powder.
In general, titanium oxide produced through the vapor-phase process exhibits excellent dispersibility as compared with titanium oxide produced through the liquid-phase process, since a solvent is not employed in the vapor-phase process.
Various vapor-phase processes for producing titanium oxide ultrafine particles have been proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 6-340423 discloses a process for producing titanium oxide through hydrolysis of titanium tetrachloride in flame, in which proportions by mol of oxygen, titanium tetrachloride, and hydrogen are regulated, and reaction of these materials is allowed to proceed, to thereby produce titanium oxide of high rutile content. Japanese Patent Application Laid-Open (kokai) No. 7-316536 discloses a process for producing a crystalline transparent titanium oxide powder having an average primary particle size of 40 nm to 150 nm, in which titanium tetrachloride is hydrolyzed at high temperature in a vapor phase, followed by rapid cooling of the resultant reaction product, wherein the flame temperature and the concentration of titanium in a raw material gas are specified. However, fine particulate titanium oxide produced through any of the above processes has high rutile content, and thus is not suitable for use in a photocatalyst or a solar cell.
Japanese Patent Application Laid-Open (kokai) No. 3-252315 discloses a vapor-phase process for producing titanium oxide containing anatase as a primary phase, in which the ratio of hydrogen in a gas mixture of oxygen and hydrogen is varied during the course of vapor-phase reaction, to thereby regulate the rutile content of the resultant titanium oxide. According to this publication, titanium oxide produced through the above process has a rutile content of 9%. However, titanium oxide disclosed in this publication has a particle size of 0.5 to 0.6 μm, which is larger than that of a typical ultrafine particle.
Ultrafine particulate titanium oxide is readily produced through a vapor-phase process employing titanium halogenide as a raw material, but halogen derived from the raw material remains in the resultant titanium oxide. Therefore, in many cases, the titanium oxide must be subjected to dehalogenation by means of heating, washing with water, or a similar technique. However, when the ultrafine particulate titanium oxide is heated in order to lower the chlorine content, sintering of titanium oxide particles proceeds, whereby the specific surface area of the titanium oxide tends to be reduced. In addition, such heating treatment may transform the crystal form of the titanium oxide from anatase to rutile. In order to prevent reduction of the specific surface area and such anatase-to-rutile transformation, the titanium oxide must be subjected to low-temperature heating or short-term heating. However, in such a case, sufficient dehalogenation of the titanium oxide fails to be attained. Japanese Patent Application Laid-Open (kokai) No. 10-251021 discloses a process for lowering the chlorine content of ultrafine particulate titanium oxide. In this process, titanium oxide is brought into contact with steam while the titanium oxide is rotated in a cylindrical rotatable heating furnace, to thereby lower the chlorine content. The titanium oxide described in this publication has a rutile content as high as 15%.
When titanium oxide particles are subjected to dehalogenation by means of, for example, washing with water, halogen remaining on the surfaces of the particles can be removed. However, halogen present in the interior of the particles tends not to be removed, since difficulty is encountered in bringing the halogen into contact with water.
As described above, low-chlorine, low-rutile ultrafine particulate titanium oxide cannot be produced through conventional vapor-phase processes.
In view of the foregoing, an object of the present invention is to provide low-halogen, low-rutile ultrafine particulate titanium oxide which is produced through a vapor-phase process and which exhibits excellent dispersibility. Another object of the present invention is to provide a process for producing the titanium oxide.